Largest-ever class action aims to expose diesel emissions

The UK’s largest-ever class action, which involves 1.5 million vehicle owners and aims to expose how many diesel cars used illegal ‘defeat devices’, is underway in the High Court.

The aim to decide what documents, said to be in the public interest, can be released, ahead of a full court case in the autumn.

Campaign group Mums for Lungs is among more than 50 organisations that want all documents to be disclosed, to expose just how many diesel cars were using illegal ‘defeat devices’ to cheat emissions tests.

The Department for Transport was ruled by the ICO in 2024 to have illegally withheld how many defeat devices operate.

The group points to research showing there are still 7.5 million Euro 5 and Euro 6 diesel cars on UK roads. "Cars certified to these standards were shown to be using defeat devices which cheat emission tests," the group says.

Separate research from the International Council on Clean Transportation suggested that 77% of Euro 6 and 85% of Euro 5 diesels have "suspicious" test results which could indicate the use of a defeat device.

Overall, there are nearly 10 million Euro 3-6 diesel cars still on UK roads, which are responsible for nearly 40% of the harmful NOx emissions produced by UK road transport.

Mums for Lungs recently protested outside the High Court and delivered an open letter to the judge calling for the disclosure of documents. This has been signed by two MPs, an NHS hospital and more than 50 charities and campaign groups.

"This looks like a sequel to the original dieselgate," says Mums for Lungs founder Jemima Hartshorn.

Cars with defeat devices are still on our roads she says, "polluting our towns and cities and making our children sick."

"We deserve to understand what is going on now – we need these documents to be made publically available due to the significant public interest in the matter."

As a result of the High Court action, the group says diesel cars from 13 leading brands could be recalled as a result of government action and the class action.

The first will involve Mercedes-Benz, Ford, Nissan/Renault and Peugeot/Citroen in October 2025.

Lawyers are also pursuing claims against Volkswagen, Porsche, BMW, Vauxhall, Fiat-Chrysler, Suzuki, JLR, Toyota, Hyundai-Kia and Mazda.

Mums for Lings also wants to see a £1bn fund from car manufacturers to pay for measures to phase out all diesel cars.  

Ask HJ

Should I swap my diesel for a petrol hybrid?

Last year in July I bought a 2024 Volvo XC90 Core 2.0 B5 diesel and I bought extra 3 year warranty so 5 in total. Today my car has 8000 miles on the clock. I have been offered a trade in option for latest 2025.5 model XC90 with same extras and everything as mine, B5 petrol option. I would have to pay in £5-6k for this new car. In your opinion should I go for this option or keep the car that I already have? My loss on my old car would be around more the less same amount as the one I would have to add now for the new one. Is the new petrol engine any good and would this upgrade would be better for the future in your opinion?

We're yet to try the updated Volvo XC90 but the changes aren't significant enough to fundamentally alter the car. Plus, the mild hybrid petrol engine will prove much less economical than the diesel engine you currently have. Although diesel is a little out of fashion, we'd argue it makes sense in a large, heavy car like the Volvo XC90, so it's a shame Volvo has dropped the option of a diesel engine in the latest version of the car. With all that in mind, we recommend you stick with your current Volvo XC90 for now.

Answered by Matt Robinson

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